His warning was reinforced by research for the Sutton Trust education charity which shows that Britain has a much more rigid society than Australia and Canada.

In a speech to the trust, Mr Clegg said it was “wrong” not to discuss “the effect of social class on prospects for social mobility”. He said: “Class still counts. We are a long distance from being a classless society.”

Mr Clegg suggested that leading colleges should lower their A-level entry grades for state school candidates.